The present invention relates to a method for forming complex-shaped sanitary fittings with a mold that includes four parts made of porous resin, and to a device for performing the method.
The expression "complex-shaped sanitary fittings" describes sanitary fittings that cannot be formed with a mold constituted by only two parts: a typical example of complex-shaped sanitary fitting is the so-called bowl or water closet.
Methods and devices for forming complex-shaped sanitary fittings are known; they differ essentially in that they operate at ambient/low pressure or at high pressure.
The ambient- or low-pressure systems are based on the use of plaster molds formed by four or five parts that are capable of self-centering by means of complementarily shaped locators. Once the mold has been closed, a ceramic suspension of kaolin, clay, et cetera, specifically termed "slip", is introduced; this suspension tends to form a dense layer on the walls of the mold due to the absorption of moisture performed by capillarity from the plaster that forms the mold. When the layer on the walls of the mold is compact enough, after approximately one hour and a quarter, the excess slip is eliminated and the mold is opened and then left to dry overnight before being able to mold another part. The long drying is indispensable to allow the plaster to perform the essential function of absorbing moisture by capillarity. In practice, therefore, it is possible to mold only one part per day per mold. Furthermore, due to its nature, the plaster of the molds tends to deteriorate rather quickly, and therefore in practice a mold can produce only 80-100 sanitary fittings, after which it is unusable.
High-pressure systems are known. These systems are based on the use of molds made of porous resin of a well-known type. This resin has a much greater mechanical strength than plaster, and is capable of withstanding relatively high operating pressures, for example 5 to 20 bar. The mold parts are unable to self-center with complementarily shaped locators, since centering is entrusted to the actuators that support the mold parts. Accordingly, each mold part is stably fixed to an actuator of its own, which opens the mold for part extraction, closes it, and also provides the force required to keep the mold closed when the slip is injected under pressure. In this manner, the time required to form a dense layer on the walls of the mold is reduced to approximately 15 minutes, and it is also not necessary to dry the mold; accordingly, it is possible to perform one molding operation after the other without pauses. However, a slip injection pressure of 10 bar generates a force of several tons which tends to open the mold. Accordingly, the closing and opening actuators, each of which is connected to one of the four or five parts of the mold, must be sized so as to withstand a considerable stress with the greatest precision. In practice, the apparatus is very large, complicated, and expensive. Furthermore, changing the molds because of wear or to change the type of production is complicated, since in order to achieve the required closing precision the mating between each mold part and the corresponding actuator must be perfect. In particular, if the first two mold parts are not well-aligned, it is impossible to close the other two.
EP 540 004, discloses a molding flask mating mechanism. However this arrangement can support only four sides of the mold, but cannot support all six sides. So the remaining unsupported two sides are charged with the inner pressure, but cannot easily withstand it. Furthermore, the urging cylinders urge on the central area of the surface of each part of the mold. Such central area, however cannot support a large amount of stress because in this area the wall of the mold is generally relatively thin. Therefore the life span of the mold is dramatically reduced.
From EP 561 613 a moulding apparatus is known. This apparatus is based on inflatable means which act on the whole surface of each part of the mold. However with this arrangement it is impossible to prevent that a large amount of the stress is discharged, also in this case, on the central area of the surface of each part of the mold. Such central area, as above said, cannot support the stress and therefore the life span of the mold is dramatically reduced. Furthermore, the mold is arranged into a container that can be opened completely. This solution is expensive, requires a complicated adjustment and is not reliable as required.
From EP 2 602 452 a process for moulding ceramic articles is known. This process involves the use of a cylindric container filled with a liquid to provide an isostatic pressure. This solution is hydraulically very complicated for sealing problems, not only inside the cylinder (between said liquid and the inner part of the mold), but also outside (the cylinder requires two large covers that must support not only the mechanical stress, but also the hydraulic seal). Furthermore the process is very slow because each time the cylinder must be closed, filled with the liquid etc. Finally this solution is very expensive because the above mentioned sealing problems require expensive means.
BE-A-887 560 corresponding to Derwent AN 81-47856D discloses an arrangement for pressing isostatically ceramic powders with extremely high pressure up to 4000 bar or more. This arrangement is extremely expensive and unsuitable for the purposes of the present invention.
Accordingly, plaster molds are still widely used even though the method is extremely slow.